In order to measure the integral scattered liminous radiation, on the one hand apparatus is known which works with an Ulbricht sphere (see Optik, 65 No. 2 (1983), pages 143-151; A plied Optics, Vol. 23, No. 21/1, November 1984, pages 3820-3825). Other apparatus is known which works with a Coblentz hemisphere (American Society for Testing Materials, ASTM designation: F, 1048-87, Standard Test Method for Measuring the Effective Surface Roughness of Optic Components by Total Integrated Scattering).
In the known methods, the sample to be examind is illuminated with a laser beam of a specific wavelength, and the light scattered by the surface of the sample is determined integrally with the Ulbricht sphere or the Coblentz hemisphere and is accepted as a measure for evaluating the surface quality of the sample.
The Ulbricht sphere is a hollow sphere which is coated on its inside with a material which scatters ideally and which is provided with a first opening for the entry of the beam of light and a second opening for the exit of the beam reflected from the surface of the sample and with a support for holding the sample in the region of the sphere wall, and it comprises a detector, disposed in the sphere wall, for the integral scattered light.
The Coblentz hemisphere is a complete or approximately complete hemisphere which is mirror-coated on its inside and is provided with a first opening for the entry of the beam of light and a second opening for the exit of the beam reflected from the sample, wherein the point of incidence of the light on the sample is offset laterally in relation to the axis of the hemisphere and the detector is disposed in the focal point of the scattered light of the sample reflected from the mirror-coated surface.
Apparatuses of the kind in question only permit examination of the surface of samples which do not have any light transmission, that is to say only of samples which are strongly reflecting or strongly absorbing.
In the case of transparent samples, the beam of light also falls on the back of the sample. There scattered light is likewise produced which is reflected from the back of the sample. This scattered light reflected from the back of the sample is superimposed on the scattered light which is reflected from the front of the sample. Thus no quantitative measurement or determination of the scattered light originating from the polished front of the sample is possible.